DESCRIPTION (Adapted from the abstract): Infections with P. aeruginosa continue to be a major cause of morbidity and mortality in immunocompromised hosts. Prevention of pseudomonal infections has been hampered by the lack of effective immunization due to toxicity and/or poor immunogenicity of polysaccharide derived vaccines in the target population of immunocompromised individuals and young children. Anti-idiotypic antibodies provide an alternative to vaccines made from bacterial products. These antibodies, directed against the antigen binding site of other antibodies, may function to mimic antigen and are capable of eliciting antigen-specific antibodies although the mechanisms of mimicry remain controversial. Two anti-idiotypic antibodies were previously produced that function as mimics of two surface polysaccharides of P. aeroginosa: one to the high molecular weight portion of the LPS O-specific side chain and the other to mucoid exopolysaccharide of mucoid strains that infect cystic fibrosis patients, These anti-idiotypes elicited P. aeroginosa-specific antibodies in mice that were able to mediated killing of the bacteria with complement, opsonize pseudomonas for killing by human neutrophils and protect mice from lethal infection. The antibodies elicited antibodies in a T-cell dependent manner. The goal of the current proposal is to determine whether the mechanism of mimicry of P. aeroginosa by our anti-idiotypes is structural and/or immunogenic. The DNA sequence of variable regions of antibodies induced by pseudomonas polysaccharide will be compared to those induced by anti-idiotype immunization. X-ray crystallography will be done to determine if antigen mimicry is due to actual structural similarity between anti-idiotypic antibodies and polysaccharide. The question of whether a peptide derived from anti-idiotypic variable regions can mimic polysaccharide antigen. Adoptive transfer experiments will be done to determine if silent clones is a cause of functional mimicry. The immunogenicity of anti-idiotype vaccines will be compared in animal models to conventional polysaccharide and polysaccharide-antigen conjugate vaccines.